1. Field of the Invention
The invention relates to an apparatus for scanning an information surface of an optical record carrier, which apparatus comprises a radiation source, an objective system for focussing a radiation beam produced by the source so as to form a radiation spot on the information surface, and a focus-error detection system for determining a deviation between the plane of focussing of the objective system and the information surface.
The phrase "scanning information" is to be understood to mean both scanning an inscribed information surface with a read beam and scanning for the purpose of recording information in said surface by means of a radiation beam whose intensity is modulated in conformity with the information to be recorded.
2. Description of the Related Art
Apparatuses for optically recording and/or reading optical record carriers have been described frequently in the literature and in patent documents. By way of example reference is made to U.S. Pat. No. 4,358,200, which relates to a read apparatus.
In order to obtain an adequate information-storage capacity, the details of the information structure in optical record carriers must have very small dimensions, of the order of micrometres, and the scanning spot should be correspondingly small. In order to form such a minute scanning spot an objective system with a comparatively large numerical aperture must be used, which objective system has a small depth of focus, of the order of 10 micrometres. Vibrations in the apparatus or wobbling motions of the record carrier may give rise to deviations between the plane of focussing of the objective system and the information surface, also referred to as focus errors, which are larger than the depth of focus, thus inhibiting a correct recording or reading process. During scanning it is therefore necessary to detect said deviations to enable the focussing to be corrected accordingly by, for example, moving the objective system.
The apparatus described in U.S. Pat. No. 4,358,200 therefore comprises a diffraction grating with a linearly varying grating period arranged in the radiation path of the scanning beam, by means of which the reflected beam which originates from the record carrier and which traverses the grating is made astigmatic. A radiation-sensitive detection system comprising four detectors is arranged between the astigmatic focal lines of this beam. In the case of a variation of the position of the information surface relative to the objective system the shape of the radiation spot formed on the detection system will change. This change in shape can be detected by combining the output signals of the detectors in the appropriate manner.
The known apparatus requires the use of a special grating. As this grating is arranged both in the path of the projected beam and in the path of the reflected beam, only a part of the radiation emitted by the source can be utilized. Moreover, very stringent requirements are imposed on the positional accuracy of the various components, in particular the detector. Finally, the overall pathlength of the beam, from the source to the detector, is comparatively large.